Recently, as the governments of all the countries of the world tighten regulations on fuel efficiency and exhaust gas of vehicles, automakers are making consistent efforts to reduce vehicle weights. Methods for reducing the vehicle weights include reduction of automotive parts through optimized design and maximized performance, use of light alternative materials such as aluminum, magnesium, engineering plastics, fiber-reinforced plastics, fiber-reinforced composite materials, etc., or the like. The use of light alternative materials can maximize the effect of reducing weights not only by reducing the weights of the parts themselves but also by allowing optimized design and parts integration.
The fiber-reinforced plastic or fiber-reinforced composite material collectively a composite material made of a plastic matrix reinforced with a glass fiber, a carbon fiber, an aramid fiber, etc. Recently, it is widely used not only in automotive interior parts but also in parts requiring high strength and heat resistance such as engine parts, chassis, etc. In addition, with the recent trend of high performance and lightweightness of parts for automobiles and industrial products, more precise shapes as well as high heat resistance, rigidity, etc. are being required for the parts.
Polyamide-based composite materials having superior rigidity, toughness, chemical resistance, etc. can replace aluminum, steel, etc. due to superior lightweightness, impact resistance, thermal expansivity, economic efficiency, etc. Because they can reduce weight up to 30% or more when used for automotive parts, they can be used not only in automotive exterior parts but also in interior parts including housing, etc. in order to reduce the weight of automobiles and allow flexible designing and easy molding, etc.
As a prior art related to the polyamide-based composite material, Korean Patent Publication No. 10-2009-0063382 relates to a reinforced polyamide resin composition, more particularly to a reinforced polyamide resin composition which experiences 5% or less of weight reduction after TGA analysis at 300° C. (after 10 minutes), has 20 g/10 min or more of fluidity measured according to ASTM D1238, has 3 or higher grade of gray scale after irradiation with xenon arc of 65 W/m2 and 126 MJ/m2 and 1,300-3,000 kg/m2 of flexural strength measured according to ASTM D790.
In order to improve the performance of the polyamide-based composite material, a compounding process of adding various reinforcing materials is necessary. But, when a large amount of glass fiber is added, the fluidity becomes unsatisfactory. And, when satisfactory physical properties are achieved, molding through existing extrusion or injection processes is difficult because of poor processability. There are limitations in improving the processability of a polymer by changing monomers or their molecular weight and molecular structure. Therefore, in order to improve the processability, an appropriate processing machine has to be selected and a fluidity control agent affecting the viscosity behavior of a molten material, such as a lubricant, has to be used.
In particular, a glass fiber-reinforced composite material with a glass fiber content of 50% or higher has limitations in productivity and physical properties due to the fracture of the glass fiber and has a flexural modulus of only 15 GPa, which falls short of 25% of that of aluminum (64 GPa). Therefore, research and development are necessary to overcome these problems.
The inventors of the present disclosure have studied on polymer compositions with superior fluidity. In doing so, they have developed a polymer composition containing 20-90 wt % of a polyamide-based resin and 10-80 wt % of a glass fiber, wherein dicyclohexylmethane bisdecanoamide is added to the polymer composition as a fluidity control agent, and have found out that the polymer composition has superior fluidity and mechanical strength.